The present invention relates to a method to assign upstream timeslots and codes to a network terminal, to a medium access controller to perform such a method, and to a network terminal.
A method of assigning upstream access codes, a medium access controller and a network terminal are already known in the art, e.g., from EP specification 0544 975. Therein a time division multiple access, hereafter abbreviated with TDMA, system is described wherein individual network terminals are assigned upstream timeslots for transmission of upstream data packets to a central station, by means of network terminal grants generated by a medium access controller incorporated within the communications network.
This method and system is, however, one-dimensional, taking only the time-division aspect into account. There is nothing mentioned how to generate a medium access control method for two-dimensional systems, where for instance both TDMA and code division multiple access, hereafter abbreviated with CDMA, are combined.
U.S. Pat. No. 5,894,473 describes a multiple access communications system and method using code and time division. The method comprises coding information signals with CDMA codewords to be transmitted over a common frequency spectrum, time compressing the CDMA code words for transmission only during allocated timeslots, activating a receiver only during the allocated timeslots to receive and decompress the time compressed CDMA codewords and decoding the decompressed CDMA codewords to recover the information signals. The allocation of the codes takes place locally in the network terminals themselves, for instance on the basis of physical level parameters such as the power level of the to be transmitted signals. U.S. Pat. No. 5,844,894 discloses that independent channel assignment strategies can be applied in different timeslots. These strategies are used between two base stations, corresponding to two central stations in the present invention, and not between the base station and the radio terminals themselves as is the subject of the present invention. Moreover, in the referenced U.S. patents, timing synchronization between the central station and the terminals is very important, as is reflected by the availability of the burst timing controller. There is also no indication within both U.S. patents how to centrally allocate these timeslots.
An aspect of the present invention is to provide a method and a medium access controller adapted to simultaneously and centrally allocate timeslots and codes to the user network stations within such a two-dimensional combined TDMA/CDMA communications network and capable to handle different degrees of synchronization between the central station and the user network terminal.
In this way, a generic two-dimensional medium access control protocol is provided wherein both the time slots as well as the codes are centrally assigned by the medium access controller to the individual network terminals, by means of a dedicated medium access control mechanism which is centralized within the network and for which a dedicated medium access control layer is used. Such a process of central assignment of timeslots allows much looser requirements of synchronization as will also become clearer from the descriptive part of this document.
In this way, the two-dimensional constraints related to the physical interferences between the network terminals, as well as between the individual communication channels are taken into account. This is extremely important for CDMA systems where these physical interferences limit the use of available codes. These physical interferences may include Rayleigh fading, and multi-user interferences.
By taking into account the total load within the network, for allocating upstream timeslots and codes, efficiency and throughput are optimized.
By letting this load to be dependent on individual requests transmitted by the terminals, these requests being indicative of the amount of upstream data packets the network terminals intend to transmit, a simple and effective method for determining the load is provided.
When the time and code allocation is dependent on a delay parameter, a compromise between throughput within the network and delay requirements pertaining to individual connections is obtained.
This delay parameter can as well be based upon the requests transmitted by the individual network terminals, or it may be generated from the connection admission control parameters, as will become clear from the descriptive part of this document.
A selection is made from a plurality of code allocation procedures. This selection and subsequent performance of the selected procedure on one hand allows to remedy specific problems within the network, while at the same time aiming to compromise parameters as throughput and fairness.
A first procedure is focussed on optimizing throughput by reducing the load within the network. A second procedure is focussed one on fairness, thereby allowing as much as possible network terminals to be served simultaneously. A third procedure represents a compromise between the previous two, and a fourth procedure guarantees delay boundaries for instance in case of guaranteed constant bit rate connections. In any of these procedures, the physical boundary conditions are taken into account.
In case some network terminals intend to transmit upstream data packets pertaining to connections for which some connection admission control parameters were negotiated during connection set-up, the second and third procedures thereby allow to take these connection admission control parameters into account. The total result is that a generic medium access control method is provided taking into account a maximum of parameters related to the physical medium, the load within the network, delay constraints and quality of service parameters.
By executing the method at predetermined time intervals, which in general correspond to the TDMA timeslots, at each timeslot a multipermit or multigrant is generated whereby thus one or more terminals are assigned one or more codes for subsequent upstream transmission of data packets to the central station.
The method is thereby not only applicable for generating grants per network terminal, but allows generating multipermits per service category queues per terminal.